Stone is one of the four resources that you will need to build your cities in Kingdoms of Camelot. It's main purpose is for the construction of buildings.
You only need small amount of stone for research and training. To produce stone you will need to build quarries in you field.
Because the main usage of stone is for building up your city is is advised that when you start your game you build plenty of quarries. You will see however that the further you progress and more and more building aren't upgradable anymore you will produce way more stone then you will need, until level 10 is achieved and you intend on building Catapults, it's wise to tear some quarries down to replace them with either Farms , sawmills or mines .It is primarily needed to upgrade a wall , level 7 needs more than a million.